Caffeine is the most common ingredient in energy drinks, but it is often combined with taurine, guarana and other ingredients. The combined effect cannot always be predicted. Says Chu, “Adding other stimulant-like ingredients while running an adrenaline-pumping trail race may create a dangerous situation that can precipitate heart palpitations.”

At the 2012 London marathon, a 30-year-old female runner died a mile from the finish line. Her cause of death was “acute cardiac failure due to extreme physical exertion complicated by the presence of DMAA.” Once an ingredient in the popular energy drink Jack 3D, DMAA (1,3-dimethylamylamine) was recently banned, but has been marketed as a dietary supplement in combination with caffeine and may be still available in some areas.

Caffeine- and taurine-containing energy drinks and strenuous physical activity have been shown to alter blood flow in the heart by constricting heart blood vessels and arteries. Taurine is an amino-acid-like substance that has been shown in labs to increase myocardial contractility (the force of the heart beat). Guarana, a natural stimulant that contains caffeine, is accepted as safe by the FDA. However, Chu warns, “Clinical case reports have implicated potentially lethal cardiac arrhythmias with the ingestion of a ‘natural energy’ guarana health drink containing a high concentration of caffeine.”

Preventive Measures

To ward off potential problems, get a physical every year and talk to your healthcare provider about your caffeine use, including doses. Experiment with caffeine during your training runs, just as you would your other nutrition choices. Before ingesting high doses of caffeine or trying new caffeine-enhanced products, consider that rescue time on the trails may be limited by geography and lack of immediate rescue personnel.